An inverter is an inverse-converting device that electrically convert direct current (DC) to alternating current (AC). The inverter used in the industry receives a power supplied from a commercial power supply, varies a voltage and frequency thereof on its own, and supplies the varied voltage and frequency to a motor. In other words, the inverter is defined as a series of devices that control the motor velocity to be used with high efficiency. The inverter may be controlled by a variable voltage variable frequency (VVVF) scheme. The inverter may vary the voltage and frequency input to the motor based on pulse width modulation (PWM) output.
FIG. 1 shows a typical inverter configuration.
Generally, an inverter 100 receives an alternating current power of three phases and a rectifier 110 rectifies the alternating current power. A direct current-link capacitor 120 may smooth and store a direct current voltage rectified by the rectifier 110. An inverting unit 130 converts the direct current voltage stored in the direct current-link capacitor 120 based on the PWM control signal. Thus, the inverting unit outputs an alternating current voltage having a predefined voltage and frequency and may provide the same to the motor. The inverting unit 130 includes three legs. In each leg, two switching elements are connected in series.
When the direct current-link voltage becomes 820 V, which is a rated voltage of the direct current-link capacitor, irrespective of the magnitude of the input voltage as input to the rectifier 110, an overvoltage fault may cause the inverter 100 to stop operating.
In a conventional case, during the operation of the inverter 100, the direct current-link voltage rarely exceeds the rated voltage of the direct current-link capacitor 120. However, depending on a local region, a fluctuation of the input voltage is large. Thus, during the operation of the inverter 100, the direct current-link voltage exceeds the rated voltage of the direct current-link capacitor 120, thereby causing the direct current-link overvoltage fault and thus causing the inverter 100 to stop.
In this case, a lifetime condition of the capacitor specified in a surge voltage test as E60384-4 capacitor test is not satisfied. Thus, there is a problem in that the capacitor may be damaged.